This disclosure relates to correction of distortion in data for digital images, and, more particularly, to an apparatus and method for correcting distortion in digital image data.
Digital imaging systems typically include one or more lenses and digital image sensors. The digital image sensors capture light from an object or scene being imaged through the lenses and process the light into electronic signals. The electronic signals are typically converted into digital signals which are representative of the light from the object or scene, and the digital signals are typically stored as digital image data in a semiconductor memory. Such digital imaging systems are commonly used in a variety of consumer, industrial and scientific applications to produce still images and/or video, including mobile telephones, digital still image and video cameras, webcams, and other such devices.
Modern image sensors are typically complementary metal oxide semiconductor (CMOS) image sensors or charge-coupled device (CCD) image sensors. The image sensors are typically arranged in a two-dimensional array of rows and columns of pixels. A typical modern digital image sensor can include millions of pixels to provide high-resolution images.
The quality of digital images, including still and video images, generated by an image sensor device can depend upon a variety of factors. For example, lens distortion can significantly affect the quality of the digital images. Lens distortion causes straight lines in a scene or object being imaged to appear curved in the image. The most common forms of distortion are radially symmetric, arising from the symmetry of the lens. Radial distortion can be classified as one of two main types: barrel distortion and pincushion distortion. In barrel distortion, image magnification decreases with distance from the optical axis. The effect is an image which appears to have been mapped around a sphere, or barrel. In pincushion distortion, image magnification increases with the distance from the optical axis. The visible effect is that lines that do not go through the center of the image are bowed inwards, toward the center of the image, like a pincushion. Complex distortion, which is a combination of barrel distortion and pincushion distortion, starts out as barrel distortion close to the image center and gradually turns into pincushion distortion toward the image periphery.
In conventional digital imaging systems, the error generated by lens distortion can be corrected by signal processing carried out in an image signal processor (ISP), which is typically formed on a semiconductor chip die. The ISP receives the digital image data from the image sensor, which is also typically formed on a semiconductor chip die. To carry out the correction of the distorted digital image data, a large buffer memory is typically required to store several hundred lines of the distorted image data. The required memory is so large, typically of multiple megabytes, that it cannot be included as part of the ISP chip. As a result, an additional memory chip, typically a dynamic random access memory (DRAM) device, is required. The additional device impacts the size and cost of the overall system. Also, the additional device introduces delays in accessing the memory and memory access bandwidth issues.